Fiber refractive index sensors play an important role in bioassays, chemical analysis and environmental monitoring. Compared to conventional refractive index sensors, fiber optic sensors offer the advantages of compact size, corrosion resistance and high sensitivity. So far, researchers have proposed many types of fiber refractive index sensors, such as surface plasmon resonance, fiber Bragg grating, long period grating and interference fiber index sensor. However, most of these sensors measure the refractive index change based on the wavelength shift of the tracking transmission spectrum. However, in practical applications, the temperature change also causes spectral drift, which interferes with the refractive index measurement, making the fiber refractive index sensor difficult to be practical. Figure 1. Dual-wavelength fiber refractive index sensing experimental device Figure 2. Test test data distribution plots and their fit curves for different solutions In response to the above problems, the research group led by Professor Xia Li from the National Research Center for Optics and Electronics and the School of Optics and Electronics Information designed an interference-type fiber refractive index sensor using dual-wavelength intensity modulation (Fig. 1). The sensor is based on an FP cavity consisting of standard single mode fiber ends. The sensing system uses two tunable lasers and a two-channel optical power meter to measure the sensor's response to the two lasers. Since the phase difference of the two lasers on the interference spectrum is constant, the response data distribution of the two wavelengths satisfies the elliptical distribution, and the refractive index of the liquid to be tested can be determined by the long axis length of the ellipse (Fig. 2). Compared with traditional sensors, temperature fluctuations only affect the distribution of test data on the elliptic curve, and do not affect the parameters of the elliptic curve, so this sensor can resist the effects of temperature fluctuations. In addition, this sensor calibration process is very convenient, and only needs to test the response of two known refractive index solutions to calibrate the sensor. It can be predicted that this sensor has a very good application prospect in the field of liquid refractive index sensing. The research results "Dual-wavelength intensity-modulated Fabry Perot refractive index sensor driven by temperature fluctuation" was published on September 1, 2018 in Optics Letters (Vol. 43, No 17, pp. 4200-4203). The work was funded by the National Natural Science Foundation of China (Project No. 61675078).
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